Add Sliding Sync `/sync` endpoint (initial implementation) (#17187)

Based on [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575): Sliding Sync

This iteration only focuses on returning the list of room IDs in the sliding window API (without sorting/filtering).

Rooms appear in the Sliding sync response based on:

 - `invite`, `join`, `knock`, `ban` membership events
 - Kicks (`leave` membership events where `sender` is different from the `user_id`/`state_key`)
 - `newly_left` (rooms that were left during the given token range, > `from_token` and <= `to_token`)
 - In order for bans/kicks to not show up, you need to `/forget` those rooms. This doesn't modify the event itself though and only adds the `forgotten` flag to `room_memberships` in Synapse. There isn't a way to tell when a room was forgotten at the moment so we can't factor it into the from/to range.

### Example request

`POST http://localhost:8008/_matrix/client/unstable/org.matrix.msc3575/sync`

```json
{
  "lists": {
    "foo-list": {
      "ranges": [ [0, 99] ],
      "sort": [ "by_notification_level", "by_recency", "by_name" ],
      "required_state": [
        ["m.room.join_rules", ""],
        ["m.room.history_visibility", ""],
        ["m.space.child", "*"]
      ],
      "timeline_limit": 100
    }
  }
}
```

Response:
```json
{
  "next_pos": "s58_224_0_13_10_1_1_16_0_1",
  "lists": {
    "foo-list": {
      "count": 1,
      "ops": [
        {
          "op": "SYNC",
          "range": [0, 99],
          "room_ids": [
            "!MmgikIyFzsuvtnbvVG:my.synapse.linux.server"
          ]
        }
      ]
    }
  },
  "rooms": {},
  "extensions": {}
}
```

1 files changed, 4 insertions, 0 deletions

diff --git a/synapse/server.py b/synapse/server.py
index 95e319d2e6..ae927c3904 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -109,6 +109,7 @@ from synapse.handlers.room_summary import RoomSummaryHandler
 from synapse.handlers.search import SearchHandler
 from synapse.handlers.send_email import SendEmailHandler
 from synapse.handlers.set_password import SetPasswordHandler
+from synapse.handlers.sliding_sync import SlidingSyncHandler
 from synapse.handlers.sso import SsoHandler
 from synapse.handlers.stats import StatsHandler
 from synapse.handlers.sync import SyncHandler
@@ -554,6 +555,9 @@ class HomeServer(metaclass=abc.ABCMeta):
     def get_sync_handler(self) -> SyncHandler:
         return SyncHandler(self)
 
+    def get_sliding_sync_handler(self) -> SlidingSyncHandler:
+        return SlidingSyncHandler(self)
+
     @cache_in_self
     def get_room_list_handler(self) -> RoomListHandler:
         return RoomListHandler(self)